Valve train component for an internal combustion engine, and method of making same

ABSTRACT

In order to provide a wear-resistant member which is excellent in wear resistance and pitting resistance, a valve train component is subjected to oxidation treatment to adjust its surface hardness to a preliminary value in a range between 550 and 800, followed by shot peening of at least a contact surface, to adjust the surface hardness to a value in a range between 800 and 1000. In a valve spring retainer, the contact surface is a seating surface against which a valve spring abuts. In a valve lifter, the contact surface is a sliding surface against which a cam lobe abuts. The components subject to the oxidation treatment are each made of a titanium alloy having an alloy composition including from 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O and the balance of Ti and unavoidable impurities.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority under 35 USC 119 based on Japanesepatent application No. 2006-091943, filed on Mar. 29, 2006. The entiredisclosure of this priority document, including specification, claimsand drawings, is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to a wear-resistant titanium member, andto methods of making same. More particularly, the present inventionrelates to a wear-resistant valve train component of a cylinder headassembly in an internal combustion engine, where the valve traincomponent is formed from an alloy containing titanium, and to methods ofmaking same.

2. Background Art

In high-performance internal combustion engines of a type used mainly inautomobiles and motorcycles, there has been a persistent demand forweight reduction in their internal component parts, in order to improvethe limit rotation speed and decrease friction, thereby enhancing theirefficiency from the viewpoints of performance improvement and reductionin environmental burden. Replacement of conventionally used steel partswith parts made of a titanium alloy material, excellent in specificstrength, has a beneficial effect of weight reduction of some of thesecomponents.

However, titanium material has a high affinity for itself or anotherelement, so that seizing occurs easily by sliding contact betweenadjacent titanium parts. Surface treatment is therefore required, forimparting such a titanium component with good sliding properties.

As the surface treatment of a titanium material, there is known atechnology of coating TiN or CrN on the surface thereof, by an ionplating method. A nonmetallic surface is available by the use of thiscoating process, so that this technology can give wear resistance to thematerial while improving its anti-seizing property (refer to, forexample, Japanese Laid-Open Patent Publication No. Hei 4-171206).

Another, different technology is also known in which shot peening isapplied to the surface of a metal material, and a filler is subsequentlyadded to the treated surface to give a compressive residual stress tothe metal material, thereby heightening the pitting resistance of thematerial, and at the same time smoothing its external surface (refer to,for example, Japanese Patent Publication No. Hei 7-8803).

According to the technology as disclosed in Japanese Laid-Open PatentPublication No. Hei 4-171206, the coating formed by the ion platingprocess has a thickness of from about 3 to 5 μm. When the coating has athickness exceeding this range, adhesion of it decreases. Such a coatingfrom 3- to 5-μm thick is formed on the relatively soft surface of thetitanium material, so that it cannot be applied to any site on thesurface because deformation of the titanium material may occur at thesliding time under high surface pressure.

Composite plating with Ni is known as similar treatment for imparting ametal surface with wear-resistance. It ensures a sufficient platingthickness of 20 μm or greater, but owing to low adhesion strength on theinterface between the titanium material and plating phase, exfoliationof the plating layer may occur after severe sliding. Thus, the compositeplating with Ni can also be applied only to a limited site.

Another surface treatment different from the above-described coating, anoxidation treatment which is, at the same time, a diffusion treatmentcan be carried out. This oxidation treatment is advantageous in cost,because it basically requires only heating of the titanium material inthe atmosphere. In addition, it is a diffusion treatment capable ofproviding good adhesion. The thickness of a hardened layer can bedetermined by selecting the treatment conditions.

In this oxidation treatment, however, the thickness of the hardenedlayer can be increased by raising the treatment temperature or treatmenttime, but it makes the surface layer brittle, and particularly itdeteriorates pitting resistance.

According to the technology disclosed in Japanese Patent Publication No.Hei 7-8803, shot peening is effective for improving the pittingresistance of a metal material, but irregularities formed on the surfaceby this treatment must subsequently be smoothed out by the applicationof a filler to the treated surface, which decreases productivity.

SUMMARY OF THE INVENTION

With the foregoing problems in view, the present invention has beenmade. An object of the present invention is to provide a wear-resistanttitanium member which is excellent in both wear resistance and pittingresistance.

In order to attain the above-described object, a wear-resistant titaniummember according to a first aspect of the present invention ischaracterized in that it is obtained by oxidation treatment, of at leastan abutting surface thereof configured for contact with another member,to adjust a surface hardness Hmv (load: 0.1 kg) to 550 or greater butless than 800, followed by shot peening to adjust the surface hardnessHmv (load: 0.1 kg) to 800 or greater but not greater than 1000.

The invention according to a second aspect hereof is characterized inthat in addition to the constitution of the first aspect, the shotpeening is performed using media having a particle size of 0.03 mm orgreater but not greater than 0.1 mm.

The invention according to a third aspect hereof is characterized inthat, in addition to the constitution of the first or second aspect, anα-case layer is formed by the oxidation treatment, the α-case layerhaving a thickness between 5 μm and 20 μm.

The invention according to a fourth aspect hereof is characterized inthat in addition to the constitution of any one of the first, second orthird aspects, the shot peening is carried out with a coverage of from100 to 500%.

The invention according to a fifth aspect hereof is characterized inthat in addition to the constitution of any one of the first throughfourth aspects, the member is made of a titanium alloy having, as analloy composition, from 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % ofO and the balance of Ti and unavoidable impurities.

The invention according to a sixth aspect hereof is characterized inthat in addition to the constitution of any one of the first throughfifth aspects, the titanium member is a valve spring retainer made oftitanium and having the abutting surface on which a valve spring abuts.

The invention according to a seventh aspect hereof is characterized inthat in addition to the constitution of any one of the first throughfifth aspects, the titanium member is a valve lifter, made of a materialcontaining titanium, and having an abutting surface over which a camlobe slides.

According to the first aspect of the invention, the wear-resistanttitanium member is obtained by oxidation treatment, of at least anabutting surface thereof with another member, to adjust a surfacehardness Hmv (load: 0.1 kg) to 550 or greater but less than 800,followed by shot peening to adjust the surface hardness Hmv (load: 0.1kg) to 800 or greater but not greater than 1000. By the oxidationtreatment which is low in cost and capable of providing good adhesion,the abutting surface with another member can have thereon a cured layerhaving a sufficient thickness, thereby having improved wear resistance.Moreover, owing to a residual stress given by the shot peeningtreatment, the material has improved pitting resistance. Thewear-resistant titanium member can therefore have a good slidingproperty even under severe sliding conditions.

According to the second aspect of the invention, the shot peening isperformed using media having a particle size of 0.03 mm or greater butnot greater than 0.1 mm. This makes it possible to suppress appearanceof irregularities on the treated surface as much as possible, therebyimproving the wear resistance further.

According to the third aspect of the invention, an α-case layer having athickness of 5 μm or greater but not greater than 20 μm is formed by theoxidation treatment.

This makes it possible to take full advantage of the effect of shotpeening, thereby improving the wear resistance and pitting resistancewhile maintaining good balance therebetween.

According to the fourth aspect of the invention, shot peening is carriedout with a coverage of from 100 to 500% so that pitting resistance canbe improved by this shot peening and owing to a compressive residualstress given thereby, a fatigue strength can also be improved. Moreover,this treatment makes it possible to maintain wear resistance whileproviding the α-case layer with a sufficient thickness.

According to the fifth aspect of the invention, the wear-resistanttitanium member is made of a titanium alloy having, as an alloycomposition, from 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O andthe balance of Ti and unavoidable impurities. This makes it possible toimprove the balance between the strength and processability and inaddition, makes it possible to form a sufficiently thick α-case layer,which is formed by oxidation treatment, in a short time compared withanother alloy and obtain the shot peening effects for improving wearresistance and pitting resistance easily.

According to the sixth aspect of the invention, the titanium member maybe a valve spring retainer, made of an alloy including titanium, andhaving an abutting surface on which a valve spring abuts. The valvespring retainer having improved wear resistance and pitting resistancecan therefore provide a good sliding property even under severe slidingconditions.

According to the seventh aspect of the invention, the titanium member isa valve lifter, made of an alloy containing titanium, and having anabutting surface over which a cam lobe slides. The valve lifter havingimproved wear resistance and pitting resistance can therefore provide agood sliding property even under severe sliding conditions.

For a more complete understanding of the present invention, the readeris referred to the following detailed description section, which shouldbe read in conjunction with the accompanying drawings. Throughout thefollowing detailed description and in the drawings, like numbers referto like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary longitudinal cross-sectional view of an internalcombustion engine incorporating a valve spring retainer according to aselected illustrative embodiment of the present invention.

FIG. 2( a) is an enlarged longitudinal cross-sectional view of a valvespring retainer.

FIG. 2( b) is an enlarged longitudinal cross-sectional view of a valvelifter.

FIG. 3 is a cross-sectional view showing the composition of the surfaceof the base material of a titanium alloy subjected to oxidationtreatment.

FIG. 4 is a schematic side view showing how to give shot peeningtreatment to a valve spring retainer.

FIG. 5 is a graph showing wear-resistance and pitting resistance as afunction of the thickness of an α-case layer before shot peeningtreatment.

FIG. 6 is a graph showing the surface hardness of the base material andthickness of the α-case layer as a function of a coverage in shotpeening treatment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A method of producing a wear-resistant case hardened titanium member,and a wear-resistant titanium member which is a product of the describedmethod will now be described relating to a selected illustrativeembodiment of the present invention, and with reference to accompanyingdrawings. It should be understood that only structures considerednecessary for clarifying the present invention are described herein.Other conventional structures, and those of ancillary and auxiliarycomponents of the system, are assumed to be known and understood bythose skilled in the art.

As illustrated in FIG. 1, an engine body 1 of a DOHC type internalcombustion engine is equipped with a cylinder block 2, having a cylinderbore 4, and with a cylinder head 3 affixed to a surface of the cylinderblock 2. A combustion chamber 6 is formed between the cylinder block 2and cylinder head 3, and the combustion chamber 6 faces a top portion ofa piston 5 fitted slidably in the cylinder bore 4.

The cylinder head 3 has an exhaust valve opening 7, which opens on anupper surface of the combustion chamber 6, and an exhaust port 8 leadingto the exhaust valve opening 7. A stem 9 a of an exhaust valve 9,provided for opening or closing the exhaust valve opening 7, is fittedslidably in a valve guide cylinder 10, disposed in the cylinder head 3.

The stem 9 a protruding from the valve guide cylinder 10 has a valvespring retainer 12 fixed at the end portion thereof via a halved cotter11 (valve keeper). A coiled valve spring 14, surrounding the stem 9 a,is disposed in a compressed state between the valve spring retainer 12and a spring seat member 13, supported by the cylinder head 3. Theexhaust valve 9 is biased in a valve closing direction by a spring forceexerted by this valve spring 14.

The cylinder head 3 also includes an inverted, cylindrically cup-shapedvalve lifter 15, which fits over and surrounds an upper portion of thestem 9 a, an upper portion of the valve spring 14 and the valve springretainer 12. The upper end of the stem 9 a is abutted concentricallyagainst an inner surface of a blocking end portion of the valve lifter15, via an inner shim 25 disposed therebetween. This valve lifter 15 isfitted slidably in a cylindrical guide hole 16 formed in the cylinderhead 3.

A valve-operating cam lobe 18, mounted on a cam shaft 17, is slidablyabutted against the outer surface of the blocking end portion of thevalve lifter 15. With the rotation of the cam shaft 17, thevalve-operating cam lobe 18 pushes the stem 9 a downwardly, via slidingcontact with the valve lifter 15, acting against the spring force of thevalve spring 14, whereby the exhaust valve 9 opens and starts itsoperation.

As illustrated in FIG. 2( a), the valve spring retainer 12 is formed asa single integral body, including a disc-shaped large-diameter portion12 a, and a small-diameter portion 12 b which is thicker in the axialdirection than the large-diameter portion 12 a and lies concentricallywith one end of the large-diameter portion 12 a. The valve springretainer 12 also includes a tapered portion 12 c, which liesconcentrically with one end of the small-diameter portion 12 b, in sucha manner that the diameter decreases with an increase in the distancefrom the small-diameter portion 12 b. The valve spring retainer 12 has aring-shaped seating surface (abutting surface) 19, defined near thejuncture between the large-diameter portion 12 a and small-diameterportion 12 b, for receiving the upper end of the valve spring 14.

The valve spring retainer 12 has a stem-fixing tapered hole 20 disposedto axially penetrate therethrough, and the halved cotter 11 fits in thetapered hole 20, in such a manner that it is sandwiched between the stem9 a to be inserted in the tapered hole 20 and the valve spring retainer12.

As illustrated in FIG. 2( b), on the other hand, the valve lifter 15includes a cylindrical portion 21, to be inserted into the guide hole 16of the cylinder head 3, and a blocking end portion 22 for closing offone end of the cylindrical portion 21. The outer surface of the blockingend portion 22 is a sliding surface (abutting surface) 23, over whichthe valve-operating cam lobe slides. The end portion 22 of the valvelifter 15 also includes a thickened central boss 24, on which the innershim 25 is abutted, formed on the inner surface thereof.

The valve spring retainer 12 and valve lifter 14 which arewear-resistant titanium members are made of a titanium alloy formed bycold forging. The titanium alloy is including from 0.5 to 1.5 wt. % ofFe, from 0.2 to 0.5 wt. % of O and the balance of Ti and unavoidableimpurities.

These valve spring retainer 12 and valve lifter 15 are subjected tooxidation treatment all over the surfaces by heating in the atmosphere.The valve spring retainer 12 and valve lifter 15 thus subjected tooxidation treatment each has a surface hardness Hmv (load: 0.1 kg) of550 or greater but less than 800.

In addition, the valve spring retainer 12 and valve lifter 15 are eachsubjected to shot peening of at least an abutting surface which providesthe sliding portions thereof for making repeated contact with anothermember during engine operation. The sliding position of the valve springretainer 12 subjected to shot peening is a sliding surface of theseating surface 19 against which the valve spring 14 is abutted, whilethe sliding position of the valve lifter 15 subjected to shot peening isthe sliding surface 23 with which the valve operating cam lobe 18 isbrought into sliding contact.

By the oxidation treatment of these valve spring retainer 12 and valvelifter 15, an α-case layer having a thickness of 5 μm or greater but notgreater than 20 μm is formed over their surfaces and by the shot peeningof the seating surface 19 and sliding surface 23, their surface hardnessHmv (load: 0.1 kg) is heightened to 800 or greater but not greater than1000.

The valve spring retainer 12 and valve lifter 15 used for an intakevalve (not illustrated) for opening or closing an intake valve opening(not illustrated) also have a similar constitution.

The valve spring retainer 12 and valve lifter 15 are case hardenedaccording to the present invention by being surface treated in themanner described below. Description will be made with the valve springretainer 12 as an example, with the understanding that the valve lifter15 is case hardened by a substantially similar process.

Oxidation treatment is given to the surface of the valve spring retainer12 formed by cool forging of a titanium alloy including from 0.5 to 1.5wt. % of Fe, from 0.2 to 0.5 wt. % of O and the balance of Ti andunavoidable impurities. Oxidation treatment is carried out by placingthe valve spring retainer 12 in an oxygen-containing atmosphere, forexample, in an atmospheric furnace and heating at from about 600 to 800°C. for several minutes to several hours.

As illustrated in FIG. 3, by this treatment, oxygen is diffused on thesurface of a base material of the valve spring retainer 12 and an oxidescale layer 31, α-case layer 32 and oxygen diffusion layer 33 are formedsuccessively in this order from the outer surface. The diffusion ofoxygen does not cause any structural change in the oxygen diffusionlayer 22, but increases its hardness over the base material before thetreatment. By the diffusion, on the other hand, the α-case layer 32 hasa thickness of 5 μm or greater but not greater than 20 μm, has a largeamount of oxygen diffused therein, and undergoes a change in thestructure of titanium to an a phase, whereby it has a higher hardnessover the base material before the treatment and exhibits excellent wearresistance. In short, by the oxidation treatment of the valve springretainer 12 made of a titanium alloy, a hardened layer including theα-case layer 21 and oxygen diffusion layer 33 is formed and the surfacehardness Hmv (load: 0.1 kg) of the titanium member becomes 550 orgreater but less than 800.

Shot peening treatment is then given to the seating surface 19 of thevalve spring retainer 12 having a surface hardness Hmv (load: 0.1 kg)adjusted to 550 or greater but less than 800 by the above-describedoxidation treatment. The shot peening treatment is carried out, forexample, by projecting media made of fine particles toward the seatingsurface 19 of the valve spring retainer 12 by using an air nozzle typeshot peening apparatus.

Arc height (shot peening intensity) and coverage (peening ratio ofmedia) are determined by controlling the conditions of shot peeningtreatment such as projection pressure, projection distance, kind ofmedia (shape, material quality), nozzle speed, work rotation speed andthe number of passes. The media including fine particles to be used forthis shot peening treatment have a particle size of preferably 0.03 mmor greater but not greater than 0.1 mm in consideration of theirregularities of the surface treated with them.

FIG. 4 illustrates one example of the shot peening treatment in themanufacturing process of the valve spring retainer 12. The shot peeningtreatment of the valve spring retainer 12 is carried out first bystacking a plurality of the valve spring retainers 12 one after anotherwith the seating surface 19 up, while passing their tapered holes 20through a supporting rod 41 which has been set up vertically.

The media are shot out from a nozzle 42 of the shot peening apparatuswhich has been inclined downward at about 45 degree while rotating thevalve spring retainers 12 together with the supporting rod 41 and arethen projected to the seating surface 19 of the uppermost valve springretainer 12.

Completion of the shot peening to the uppermost valve spring retainer 12is followed by the relative movement of the supporting rod 41 and nozzle42 and then shot peening of the valve spring retainers 12 is carried outsuccessively from the upper one to the lower one.

By the shot peening treatment of the valve spring retainer 12 in theabove-described manner, the oxide scale layer 31, which is a scale onthe treated surface, is removed. The projection of fine particlesincreases the hardness of the α-case layer 32, thereby increasing itssurface hardness Hmv (load: 0.1 kg) to 800 or greater but not greaterthan 1000. At the same time, it imparts the retainer with a compressiveresidual stress and thereby heightens the fatigue strength, whereby theretainer has improved pitting resistance.

When the media are projected to the seating surface 19 of the valvespring retainer 12 from the oblique direction as described above, theyare also projected to a corner 19 a between the seating surface 19 and awall portion adjacent to the seating surface 19, which greatly enhancesthe fatigue strength at the corner 19 a.

FIG. 5 is a graph showing the wear resistance and pitting resistancebefore shot peening treatment as a function of the thickness of theα-case layer 32. As illustrated in FIG. 5, the valve spring retainer 12made of a titanium alloy has improved pitting resistance, but hasreduced wear resistance as the α-case layer 32 becomes thinner. On thecontrary, with an increase in the thickness of the α-case layer 32, ithas improved wear resistance but has reduced pitting resistance. Such aphenomenon is presumed to occur because the surface becomes fragile withan increase in the thickness of the α-case layer 32 and pittingoriginates from minute cracks.

Shot peening treatment after the oxidation treatment imparts the α-caselayer 32 with a residual stress, which results in great improvement inpitting resistance. Deterioration in the pitting resistance cantherefore be suppressed even if the thickness becomes greater. Aftershot peening treatment, even if the α-case layer 32 has a thickness of20 μm, pitting resistance is substantially equal to that when the α-caselayer 32 has a thickness of 5 μm or less. Moreover, with an increase inthe surface hardness brought by the shot peening treatment, the wearresistance also improves greatly.

When the α-case layer 32 is thicker than 20 μm, even shot peeningtreatment cannot improve the pitting resistance sufficiently because ofthe breaking tendency of the grain boundary of recrystallized crystalgrains. The thickness of the α-case layer 32 is therefore preferably 5μm or greater but not greater than 20 μm in consideration of the wearresistance and pitting resistance.

FIG. 6 is a graph showing the surface hardness of the base material andthickness of the α-case layer 32 as a function of the coverage in theshot peening treatment. The coverage smaller than 100% makes itdifficult to attain sufficient surface hardness, because improvingdegrees of pitting resistance and fatigue strength attributable to thecompressive residual strength also brought by the shot peening treatmentare small. When the coverage exceeds 500%, on the other hand, the α-caselayer 32 is shaved to be thin, which leads to deterioration in wearresistance. The coverage in the shot peening treatment therefore fallswithin a range of from 100 to 500%.

Surface treatment given to the valve lifter 15 is substantially similarto that employed for the valve spring retainer 12. In the case of thevalve lifter 15, the shot peening treatment after the oxidationtreatment also heightens the surface hardness Hmv (load: 0.1 kg) on thesliding surface 23 to 800 or greater but not greater than 1000 and as aresult, it has improved wear resistance and pitting resistance.

The titanium alloy employed here has an alloy composition including from0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O and the balance of Tiand unavoidable impurities. The alloy composition within theabove-described range makes it possible to improve the balance betweenthe strength and processability, makes it possible to form asufficiently thick α-case layer by the oxidation treatment in a shorttime compared with another alloy, and facilitates obtaining the effectsof shot peening for improving wear resistance and pitting resistance.

As described above, in the valve spring retainer 12 and valve lifter 15made of titanium according to this Embodiment, the seating surface 19against which the valve spring 14 abuts and the sliding surface 23against which the valve operating cam lobe 18 abuts are adjusted to havea surface hardness Hmv (load: 0.1 kg) of 550 or greater but less than800 by the oxidation treatment and then adjusted to have a surfacehardness Hmv (load: 0.1 kg) of 800 or greater but not greater than 1000by the shot peening treatment. This makes it possible to obtain a curedlayer having a sufficient thickness and including the α-case layer 32and oxygen diffusion layer 33 by the oxidation treatment which is low incost and exhibits good adhesion, thereby improving wear resistance. Atthe same time, pitting resistance can be improved owing to a residualstress given by the shot peening treatment. As a result, the valvespring retainer 12 and valve lifter 15 made of titanium and having agood sliding property even under severe sliding conditions can beobtained.

Shot peening with media having a particle size of 0.03 mm or greater butnot greater than 0.1 mm makes it possible to suppress the formation ofirregularities on the treated surface as much as possible, therebyimproving the wear resistance further.

Moreover, since the α-case layer 32 having a thickness of 5 μm orgreater but not greater than 20 μm is formed by the oxidation treatment,it is possible to improve wear resistance and pitting resistance in agood balance by making the best use of the effects of shot peening.

The shot peening is carried out with a coverage of from 100 to 500%.This makes it possible to improve the pitting resistance and the fatiguestrength attributable to the compressive residual stress also brought byshot peening, and at the same time, maintain the wear resistance whileproviding the α-case layer 32 with a sufficient thickness.

By using a titanium alloy having an alloy composition including from 0.5to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O and the balance of Ti andunavoidable impurities, the balance between the strength andprocessability can be improved and at the same time, a sufficientlythick α-case layer 32, which is formed by the oxidation treatment, canbe formed in a short time compared with another alloy, whereby effectsof shot peening for improving the wear resistance and pitting resistanceare available easily.

The present invention is not limited to the above-described embodiment,but can be changed or modified as needed.

In this Embodiment, a valve spring retainer and valve lifter weredescribed as examples of the wear-resistant titanium member, but theinvention is not limited to these members but can be applied to variousproducts including engine parts such as crank shaft and rocker shaft.

In the valve lifter according to this Embodiment, shot peening is givento the sliding surface with a cam, but shot peening may be given to notonly this sliding surface but also another sliding surface with a guidehole of a cylinder head.

The titanium member of the present invention is not limited to thathaving the above-described composition, but Ti-6Al-4V alloy, Ti-3Al-2.5Valloy or pure titanium of JIS 2 grade may be employed instead.

EXAMPLES (Test 1)

Tests were made on the influence of the conditions of oxidationtreatment and presence or absence of shot peening treatment on the wearresistance and pitting resistance of a valve spring retainer. As testspecimens, valve spring retainers were made by cool forging of a rodmade of a Ti-1Fe-0.30 (wt. %) alloy and having a diameter of 10 mm.Oxidation treatment was then given to them under varied treatmentconditions as follows: 500° C.×5 hours, 600° C.×5 hours, 700° C.×5hours, 800° C.×5 hours and 900° C×5 hours. Valve spring retainers to bedisposed on the abutting surface with a valve spring were subjected toshot peening further. Shot peening was performed with “#400 High SpeedSteel” as media under the following conditions: distance of 200 mm,projection pressure of 0.5 MPa and 300% of coverage.

Surface hardness was determined by measuring it at ten points under aload of 100 g by a Micro Vickers hardness tester after removal ofirregularities from the surface by buffing with alumina polishingparticles of 0.3 μm in size and then averaging the measured values ateight points except the maximum and minimum values. Wear resistance andpitting resistance were evaluated based on the results of a motoringdurability test made on a valve spring having, in the vicinity of theend portion thereof, a burr. The results of motoring durability test areshown in Table 1.

TABLE 1 Oxidation Without shot Surface hardness treatment peening Withshot peening after conditions Wear Pitting Wear Pitting shot (Hmv 0.1)500° C. × 5 hrs C — C — 628 600° C. × 5 hrs C B A A 801 700° C. × 5 hrsA B A A 876 800° C. × 5 hrs A C A A 992 900° C. × 5 hrs — — — — —

In table 1, “C” means appearance of severe wear or pitting, “B” meansappearance of wear or pitting, and “A” means exhibition of a goodsliding property without appearance of wear or pitting. These resultshave revealed that the valve spring retainers having both wearresistance and pitting resistance are those subjected to oxidationtreatment under the conditions of from 600° C.×5 hours to 800° C.×5hours, followed by shot peening treatment. These valve spring retainershad a surface hardness Hmv (load: 0.1 kg) of 800 or greater but notgreater than 1000.

The valve spring retainer subjected to oxidation treatment under thecondition of 900° C.×5 hours, on the other hand, was not evaluatedbecause its surface was markedly coarsened owing to a large amount ofoxide scales generated after the oxidation treatment.

(Test 2)

A test on the wear resistance and pitting resistance of a valve springretainer was conducted while changing the diameter of media to be usedfor shot peening treatment. Test specimens were, similar to Example 1,valve spring retainers manufactured by cool forging of a rod made of aTi-1Fe-0.30 (wt. %) alloy and having a diameter of 10 mm. The resultingtest specimens were subjected to oxidation treatment under theconditions of 700° C.×5 hours, followed by shot peening while changingthe diameter of a media. As in Test 1, evaluation was made based on theresults of the motoring durability test. Conditions are similar to thoseemployed in Example 1 except that the diameter of the media was changed.The results of the motoring durability test are shown in Table 2.

TABLE 2 Diameter of media Wear Pitting 0.03 A A 0.05 A A 0.10 A A 0.30 BB 0.60 C C

It has been found that the test specimens have good wear resistance andpitting resistance when media having a particle size of 0.03 mm orgreater but not greater than 0.1 mm are used, while use of media havinga particle size exceeding 0.3 mm do not lead to good results because itgives a damage to the cured layer including an α-case layer and oxygendiffusion layer formed by the oxidation treatment.

(Test 3)

Tests on the wear resistance and pitting resistance of a valve springretainer were made while changing the thickness of the α-case layer. Thethickness of the α-case layer varies, depending on the conditions ofoxidation treatment. Test specimens were, similar to Example 1, valvespring retainers manufactured by cool forging of a rod made of aTi-1Fe-0.30 (wt. %) alloy and having a diameter of 10 mm. The resultingtest specimens were subjected to oxidation treatment under variousconditions, followed by shot peening under similar conditions to thoseemployed in Test 1. After the motoring durability test of thethus-prepared valve spring retainers as in Test 1, each retainer wascut, buried in a polishing resin and polished. Then, the thickness ofthe α-case layer on the surface was observed. The α-case layer is aportion of a titanium structure changed to an cc phase and becoming ahard layer by the oxygen penetrated from the surface and diffused in themember and it looks white on a microscope owing to etching. No change instructure is found even inside of the α-case layer but a so-calledoxygen diffusion layer, that is, a layer having an increased hardness bythe diffusion of oxygen exists. The results of the motoring durabilitytest are shown in Table 3.

TABLE 3 Thickness of α case Wear Pitting 2 C A 5 A A 11 A A 20 A A 24 AC

As a result, when the α-case layer had a thickness of 2 μm, theresulting member was inferior in wear resistance itself, while when theα-case layer had a thickness of 24 μm, pitting appeared because theboundary of recrystallized crystal grains broke easily. Test resultswere good when the α-case layer had a thickness of 5 μm or greater butnot greater than 20 μm.

(Test 4)

Next, tests were made on the influence of the conditions of oxidationtreatment and presence or absence of shot peening treatment on the wearresistance and pitting resistance of a valve lifter. Specimens used forthe test were valve lifters manufactured by forging and mechanicalprocessing of a rod made of a Ti-1Fe-0.30 (wt. %) alloy and having adiameter of 30 mm. The specimens were subjected to oxidation treatmentunder varied treatment conditions as follows: 500° C.×5 hours, 600° C.×5hours, 700° C.×5 hours, and 800° C.×5 hours. The results of Test 1showed that oxidation treatment under the condition of 900° C.×5 hourswas not worthy of evaluation so that oxidation treatment under thiscondition was omitted. Valve lifters subjected to shot peening furtherat the sliding surface thereof with a cam were also prepared. Themeasuring method of surface hardness and shot peening conditions weresimilar to those employed in Test 1.

The wear resistance and pitting resistance of the valve lifters wereevaluated in accordance with a motoring durability test using a camshaft having a cam width reduced by 25%. The results of the motoringdurability test are shown in Table 4.

TABLE 4 Oxidation Without shot Surface hardness treatment peening Withshot peening after conditions Wear Pitting Wear Pitting shot (Hmv 0.1)500° C. × 5 hrs C — C — 616 600° C. × 5 hrs C A A A 800 700° C. × 5 hrsA B A A 869 800° C. × 5 hrs A C A A 993

In table 4, “C” means appearance of severe wear or pitting, “B” meansappearance of wear or pitting, and “A” means exhibition of a goodsliding property without appearance of wear or pitting. These resultshave revealed that the valve lifters having both wear resistance andpitting resistance are those subjected to oxidation treatment under theconditions of from 600° C.×5 hours to 800° C.×5 hours, followed by shotpeening treatment. These valve lifters had a surface hardness Hmv (load:0.1 kg) of 800 or greater but not greater than 1000. Thus, the valvelifters showed similar results to those of the valve spring retainers.

(Test 5)

Tests on the wear resistance and pitting resistance of the valve lifterswere made while changing the diameter of media to be used for shotpeening treatment. Test specimens were, similar to Example 4, valvelifters manufactured by forging and mechanical processing of a rod madeof a Ti-1Fe-0.30 (wt. %) alloy and having a diameter of 10 mm. Theresulting test specimens were subjected to oxidation treatment under theconditions of 700° C.×5 hours, followed by shot peening while changingthe diameter of media. As in Test 4, evaluation was made based on theresults of a motoring durability test. Conditions are similar to thoseemployed in Example 4 except that the diameter of the media is changed.The results of the motoring durability test are shown in Table 5.

TABLE 5 Diameter of media Wear Pitting 0.03 A A 0.05 A A 0.10 A A 0.30 BB 0.60 C C

It has been found that the test specimens shows good wear resistance andpitting resistance when media having a particle size of 0.03 mm orgreater but not greater than 0.1 mm are used, while use of media havinga particle size exceeding 0.3 mm do not lead to good results because itgives a damage to the cured layer including an α-case layer and oxygendiffusion layer formed by the oxidation treatment. Thus, the results ofthe valve lifters were similar to those of the valve spring retainers.

(Test 6)

Tests on the wear resistance and pitting resistance of valve lifterswere made while changing the thickness of the α-case layer by changingthe conditions of oxidation treatment. Test specimens were, similar toExample 1, valve spring retainers manufactured by cool forging of a rodmade of a Ti-1Fe-0.30 (wt. %) alloy and having a diameter of 10 mm. Theresulting valve spring retainers were subjected to oxidation treatmentunder various conditions, followed by shot peening under similarconditions to those employed in Test 1. After the motoring durabilitytest of the thus-prepared valve spring retainers as in Test 1, eachretainer was cut, buried in a polishing resin and polished. Then, thethickness of the α-case layer of it was observed. The results of themotoring durability test are shown in Table 6.

TABLE 6 Thickness of α case Wear Pitting 2 C A 5 A A 11 A A 20 A A 24 AC

As a result, when the α-case layer had a thickness of 2 μm, theresulting member was inferior in wear resistance itself, while when theα-case layer had a thickness of 24 μm, pitting appeared because theboundary of recrystallized crystal grains broke easily. Test resultswere good when the α-case layer had a thickness of 5 μm or greater butnot greater than 20 μm. Thus, the evaluation results of the valvelifters were also similar to those of the valve spring retainers.

(Test 7)

Tests were made while changing the alloy composition of the valve springretainers or valve lifters. Those having an alloy composition includingfrom 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O, and the balanceof Ti and unavoidable impurities were employed. It has been found thatwhen the Fe and O contents falls within the above-described ranges,respectively, tensile strength increases almost in proportion to theiramounts. Within the above-described ranges, however, a difference infatigue strength is not so large as that in tensile strength, meaningthat this composition range is advantageous for mass production control.It has also been found that the advantage of the present invention canbe utilized more within this composition range because an alloy havingthe above-described composition range enables formation of a thickerα-case layer compared with another alloy.

(Test 8)

Tests were made on the influence of the conditions of oxidationtreatment and presence or absence of shot peening treatment on the wearresistance and pitting resistance by using a Fabry wear tester. Fabrywear test specimens were prepared by mechanical processing of a rod madeof a Ti-1Fe-0.30 (wt. %) alloy and having a diameter of 10 mm. Thespecimens were subjected to oxidation treatment under varied treatmentconditions as follows: 500° C.×5 hours, 600° C.×5 hours, 700° C.×5hours, 800° C.×5 hours, and 900° C.×5 hours. Specimens subjected to shotpeening treatment further were also prepared. In the wear test, wearresistance was evaluated when the test specimen was used on the blockside, while pitting resistance was evaluated when the test specimen wasused on the pin side. A partner material employed here was an SCMcarburized material having a surface hardness Hmv (load: 0.1 kg) ofabout 750. The measuring method of the surface hardness and shot peeningconditions employed for the test specimens were similar to thoseemployed in Test 1. The test results are shown in Table 7.

TABLE 7 Oxidation Without shot Surface hardness treatment peening Withshot peening after conditions Wear Pitting Wear Pitting shot (Hmv 0.1)None C — C 459 500° C. × 5 hrs C — C — 623 600° C. × 5 hr B B A A 811700° C. × 5 hrs A B A A 876 800° C. × 5 hrs A C A A 996 900° C. × 5 hrs— — — — —

In table 7, “C” means appearance of severe wear or pitting, “B” meansappearance of wear or pitting, and “A” means exhibition of a goodsliding property without appearance of wear or pitting. These resultshave revealed that the test specimens having both wear resistance andpitting resistance are those subjected to oxidation treatment under theconditions of from 600° C.×5 hours to 800° C.×5 hours, followed by shotpeening treatment. These test specimens have a surface hardness Hmv(load: 0.1 kg) of 800 or greater but not greater than 1000. The testspecimen subjected to oxidation treatment under the condition of 900°C.×5 hours, on the other hand, was not evaluated because its surface wasmarkedly coarsened owing to a large amount of oxide scales generatedafter the oxidation treatment.

(Test 9)

Tests on the wear resistance and pitting resistance of a valve lifterwere conducted while changing the diameter of media to be used for shotpeening treatment. As in Test 8, Fabry wear test specimens were preparedby mechanical processing of a rod made of a Ti-1Fe-0.30 (wt. %) alloyand having a diameter of 10 mm. The resulting test specimens wassubjected to oxidation treatment under the conditions of 700° C.×5hours, followed by shot peening while changing the diameter of media.Influence of it was evaluated by the wear test. Conditions are similarto those employed in Example 1 except that the diameter of the media ischanged. The test results are shown in Table 8.

TABLE 8 Diameter of media Wear Pitting 0.03 A A 0.05 A A 0.10 A A 0.30 BB 0.60 B C

It has been found that the test specimens have good wear resistance andpitting resistance when media having a particle size of 0.03 mm orgreater but not greater than 0.1 mm are used, while use of media havinga particle size exceeding 0.3 mm do not lead to good results because itgives a damage to the cured layer itself formed by the oxidationtreatment.

(Test 10)

Tests on the wear resistance and pitting resistance were made using aFabry wear tester while changing the thickness of the α-case layer bychanging the conditions of oxidation treatment. Fabry wear testspecimens were made in a similar manner to that employed in Test 8 bythe mechanical processing of a rod made of a Ti-1Fe-0.30 (wt. %) alloyand having a diameter of 10 mm. The resulting test specimens weresubjected to oxidation treatment under various conditions, followed byshot peening treatment under similar conditions to those employed inTest 8. After the wear test of the test specimens, they were each cut,buried in a polishing resin and polished. Then, the thickness of theα-case layer on the surface was observed. The test results are shown inTable 9.

TABLE 9 Thickness of α case Wear Pitting 2 B A 5 A A 11 A A 20 A A 24 AC

As a result, when the α-case layer had a thickness of 2 μm, the specimenwas inferior in wear resistance itself, while when the α-case layer hada thickness of 24 μm, pitting appeared because the boundary ofrecrystallized crystal grains broke easily. Test results were good whenthe α-case layer had a thickness of 5 μm or greater but not greater than20 μm.

(Test 11)

A test was made using Ti-6Al-4V alloy, Ti-3Al-2.5V alloy and puretitanium of JIS 2 grade under similar conditions to those employed inExample 8 except that oxidation treatment was conducted under theconditions of 700° C.×5 hours. As a result, any of these titaniummaterials showed good wear resistance and pitting resistance whensubjected to shot peening. They had a surface hardness Hmv (load: 0.1kg) of 910, 884, and 818, respectively, after shot peening.

(Test 12)

Shot peening was given, under similar conditions to those employed inExample 4, to a rocker shaft made of a Ti-6Al-4V alloy and used afteroxidation treatment and a bench durability test on the resulting rockershaft was conducted. The oxidation treatment is ordinarily performed at600° C. for 6 hours. The rocker shaft of the present invention showeddurability ten times as much as that of the conventional one in theultimate durability test, showing that the invention has a great effecton a practical part.

Although the present invention has been described herein with respect toa number of specific illustrative embodiments, the foregoing descriptionis intended to illustrate, rather than to limit the invention. Thoseskilled in the art will realize that many modifications of theillustrative embodiment could be made which would be operable. All suchmodifications, which are within the scope of the claims, are intended tobe within the scope and spirit of the present invention.

1. A wear-resistant valve train component member formed from a materialcomprising titanium, said valve train component member being a productof a process including steps of: case-hardening at least one surface ofsaid member, designated as an abutting surface configured for abuttingcontact with another member, by oxidation treatment to adjust a surfacehardness Hmv (load: 0.1 kg) of said surface to an oxidized value in arange between 550 and 800 Hmv; followed by shot peening said abuttingsurface to adjust the surface hardness Hmv (load: 0.1 kg) of saidsurface to a final value in a range between 800 and 1000 Hmv.
 2. Awear-resistant valve train component according to claim 1, wherein theshot peening is performed using media having a particle size in a rangefrom about 0.03 mm to about 0.1 mm.
 3. A wear-resistant valve traincomponent according to claim 1, wherein an α-case layer, having athickness in a range from about 5 μm to about 20 μm, is formed by theoxidation treatment.
 4. A wear-resistant valve train component accordingto claim 1, wherein the shot peening is carried out with a coverage offrom 100 to 500%.
 5. A wear-resistant valve train component memberaccording to claim 1, wherein the member is made of a titanium alloyhaving, as an alloy composition, from 0.5 to 1.5 wt. % of Fe, from 0.2to 0.5 wt. % of O and the balance of Ti and unavoidable impurities.
 6. Awear-resistant valve train component member according to claim 1,wherein the member is a valve spring retainer having an abutting surfaceon which a valve spring abuts.
 7. A wear-resistant valve train componentmember according to claim 1, wherein the member is a valve lifter havingan abutting surface on which a cam lobe slides.
 8. A wear-resistantvalve train component member formed from a material comprising titanium,said valve train component member being a product of a process includingsteps of: case-hardening at least one surface of said member, designatedas an abutting surface configured for abutting contact with anothermember, by oxidation treatment to adjust a surface hardness Hmv (load:0.1 kg) of said surface to an oxidized value in a range between 550 and800 Hmv; followed by shot peening said abutting surface to adjust thesurface hardness Hmv (load: 0.1 kg) of said surface to a final value ina range between 800 and 1000 Hmv; wherein the shot peening is performedusing media having a particle size in a range from about 0.03 mm toabout 0.1 mm; and wherein an α-case layer, having a thickness in a rangefrom about 5 μm to about 20 μm, is formed by the oxidation treatment. 9.A wear-resistant valve train component according to claim 8, wherein theshot peening is carried out with a coverage of from 100 to 500%.
 10. Awear-resistant valve train component member according to claim 9,wherein the member is made of a titanium alloy having, as an alloycomposition, from 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O andthe balance of Ti and unavoidable impurities.
 11. A wear-resistant valvetrain component member according to claim 8, wherein the member iseither a valve spring retainer or a valve lifter.
 12. A method ofcase-hardening a valve train component member formed from a materialcomprising titanium, said method including the steps of: case-hardeningat least one surface of said member, designated as an abutting surfaceconfigured for abutting contact with another member, by oxidationtreatment to adjust a surface hardness Hmv (load: 0.1 kg) of saidsurface to an oxidized value in a range between 550 and 800 Hmv; ansubsequently, shot peening said abutting surface to adjust the surfacehardness Hmv (load: 0.1 kg) of said surface to a final value in a rangebetween 800 and 1000 Hmv.
 13. A method of case-hardening a valve traincomponent member according to claim 12, wherein the shot peening isperformed using media having a particle size in a range from about 0.03mm to about 0.1 mm.
 14. A method of case-hardening a valve traincomponent member according to claim 12, wherein an α-case layer, havinga thickness in a range from about 5 μm to about 20 μm, is formed by theoxidation treatment.
 15. A method of case-hardening a valve traincomponent member according to claim 12, wherein the shot peening iscarried out with a coverage of from 100 to 500%.
 16. A method ofcase-hardening a valve train component member according to claim 12,wherein the member is made of a titanium alloy having, as an alloycomposition, from 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O andthe balance of Ti and unavoidable impurities.
 17. A method ofcase-hardening a valve train component member according to claim 12,wherein the member is a valve spring retainer having an abutting surfaceon which a valve spring abuts.
 18. A method of case-hardening a valvetrain component member according to claim 12, wherein the member is avalve lifter having an abutting surface on which a cam lobe slides.